High data rate multilevel clock recovery system

ABSTRACT

Digital receiver systems and clock recovery techniques for use in digital receiver systems are provided to implement asynchronous baud-rate clock recovery systems for high data rate serial receivers multilevel line modulation. A two-stage postcursor ISI equalization system is provided to efficiently emulate a 4-level DFE (decision feedback equalization) system, for example, while converting a 4-level equalized signal to s 2-level equalized signal. For example, a two stage postcursor ISI equalization system includes a DFE stage which operates on a most significant component of a given 4-level data symbol, followed by a DFFE (decision-feedforward equalizer) stage which operates on a least significant component of the given 4-level data symbol. In parallel with the DFFE stage, an estimate of the least significant component is subtracted from the equalized 4-level data symbol to convert the 4-level data symbol to a 2-level symbol.

TECHNICAL FIELD

This disclosure generally relates to digital receiver systems and, morespecifically, to clock recovery techniques for use in digital receiversystems.

BACKGROUND

Current state of the art integrated circuit CMOS (complementary metaloxide semiconductor) technology supports the implementation ofdigital-based receiver systems for use in high data rate (e.g. 28 Gb/sand higher) wireline communications systems. These wirelinecommunications systems include, for example, backplane transceivers usedin data processing systems and networking infrastructure such asinternet-protocol (IP) routers and switches. Some advantages of using adigital-based receiver design include, for example, the ability torealize digital functions in low power and low die area using state ofthe art CMOS processes such as 14 nm CMOS and beyond. Further,digital-based receiver designs can efficiently support line modulationschemes that are more complex than the standard 2-level “Non-Return toZero” (NRZ) modulation scheme that is currently used in a wide majorityof backplane wireline communication transceivers up to data rates of 28Gb/s. Indeed, evolving industry standards are now migrating to 4-levelline modulation to improve the spectral efficiency, or equivalently, toincrease the maximum data rate than can be transmitted through abandlimited wireline channel. By way of example, the current 56 Gb/sindustry data rate for medium-reach electrical backplane applications issupported by 4-level PAM (pulse amplitude modulation). A digitalreceiver design is ideal to realize a 56 Gb/s 4-level PAM datatransceiver system because such design can readily implement complexdigital functions, such as line equalization and digital clock recoveryfunctions, which are necessary to accurately process and decode 4-levelmodulated signals.

SUMMARY

Embodiments of the invention include digital receiver systems and clockrecovery techniques for use in digital receiver systems. For example,one embodiment includes a clock recovery system. The clock recoverysystem comprises an ADC (analog-to-digital converter) circuit, a FFE(feed forward equalizer) circuit, a DFE (decision feedback equalizer)circuit, a DFFE (decision feedforward equalizer) circuit, and controlcircuitry. The ADC circuit is configured to sample an analog signal inresponse to sample a clock signal and generate a digital signalcomprising sequence of digital data samples. The analog signal comprisesa stream of data symbols, wherein each data symbol comprises a pluralityn of symbol components, including at least a first symbol component anda second symbol component. Each digital data sample corresponds to onedata symbol in the analog signal. The FFE circuit is configured toremove precursor ISI (intersymbol interference) from the digital datasamples and at least partially remove postcursor ISI from the digitaldata samples, and output FFE equalized data samples. The DFE circuit isconfigured to remove a first component of postcursor ISI from the FFEequalized data samples, and output DFE equalized data samples, whereinthe first component of postcursor ISI of a given data sample arises fromthe first symbol component of a previous data sample. The DFFE circuitconfigured to remove a second component of postcursor ISI from the DFEequalized data samples, and generate DFFE equalized data samples,wherein the second component of postcursor ISI of a given data samplearises from the second symbol component of a previous data sample. TheDFFE circuit is further configured to remove all but one of the n symbolcomponents from each DFFE equalized data sample, and output an equalized2-level digital signal comprising a sequence of equalized 2-level datasamples. The control circuitry is configured to process the equalized2-level digital signal to determine an amount of any residual postcursorand precursor ISI of the equalized 2-level digital signal, and to adjusta phase of the sample clock signal that is applied to the ADC circuitryto minimize the residual postcursor and precursor ISI of the equalized2-level digital signal.

In one embodiment of the invention, the analog signal comprises 4-levelPAM signal, wherein each data symbol comprises a most-significant bit(MSB) as the first symbol component, and a least significant bit (LSB)as the second symbol component, and wherein the equalized 2-leveldigital signal output from the DFFE circuit comprises only the MSBcomponents of the data samples.

Another embodiment of the invention includes a digital receiver system.The digital receiver system includes an amplifier circuit, an ADCcircuit, a digital equalization system, and a clock recovery system. Theamplifier circuit is configured to amplify a received analog signal,wherein the analog signal comprises a stream of data symbols, whereineach data symbol comprises a plurality (n) of symbol components,including at least a first symbol component and a second symbolcomponent. The ADC circuit is configured to sample the analog signal inresponse to a sample clock signal and generate a digital signalcomprising sequence of digital data samples, wherein each digital datasample corresponds to one data symbol in the analog signal. The digitalequalization system is configured to filter the digital signal to removeintersymbol interference and recover the stream of data symbols. Theclock recovery system is configured to process the digital signal andgenerate the sample clock signal which is applied to the ADC circuit.The clock recovery system includes a FFE circuit, a DFE circuit, a DFFEcircuit, and control circuitry. The FFE circuit is configured to removeprecursor ISI (intersymbol interference) from the digital data samplesand at least partially remove postcursor ISI from the digital datasamples, and output FFE equalized data samples. The DFE circuit isconfigured to remove a first component of postcursor ISI from the FFEequalized data samples, and output DFE equalized data samples, whereinthe first component of postcursor ISI of a given data sample arises fromthe first symbol component of a previous data sample. The DFFE circuitconfigured to remove a second component of postcursor ISI from the DFEequalized data samples, and generate DFFE equalized data samples,wherein the second component of postcursor ISI of a given data samplearises from the second symbol component of a previous data sample. TheDFFE circuit is further configured to remove all but one of the n symbolcomponents from each DFFE equalized data sample, and output an equalized2-level digital signal comprising a sequence of equalized 2-level datasamples. The control circuitry is configured to process the equalized2-level digital signal to determine an amount of any residual postcursorand precursor ISI of the equalized 2-level digital signal, and to adjusta phase of the sample clock signal that is applied to the ADC circuitryto minimize the residual postcursor and precursor ISI of the equalized2-level digital signal.

Another embodiment of the invention includes a clock recovery method,which comprises: receiving an analog signal, wherein the analog signalcomprises a stream of data symbols, wherein each data symbol comprises aplurality (n) of symbol components, including at least a first symbolcomponent and a second symbol component; converting the analog signalinto a digital signal by sampling the analog signal using a sample clocksignal to generate a sequence of digital data samples, wherein eachdigital data sample corresponds to one data symbol in the analog signal;equalizing the digital signal using a FFE (feed forward equalization)function to remove precursor ISI (intersymbol interference) from thedigital data samples and at least partially remove postcursor ISI fromthe digital data samples, and generate FFE equalized data samples;equalizing the FFE equalized data samples using a DFE (decision feedbackequalization) function to remove a first component of postcursor ISIfrom the FFE equalized data samples, and generate DFE equalized datasamples, wherein the first component of postcursor ISI of a given datasample arises from the first symbol component of a previous data sample;equalizing the DFE equalized data samples using a DFFE (decisionfeedforward equalization) function to remove a second component ofpostcursor ISI from the DFE equalized data samples, and generate DFFEequalized data samples, wherein the second component of postcursor ISIof a given data sample arises from the second symbol component of aprevious data sample; removing all but one of the n symbol componentsfrom each DFFE equalized data sample to generate an equalized 2-leveldigital signal comprising a sequence of equalized 2-level data samples;and processing the equalized 2-level digital signal to determine anamount of any residual postcursor and precursor ISI of the equalized2-level digital signal, and to adjust a phase of the sample clock signalto minimize the residual postcursor and precursor ISI of the equalized2-level digital signal.

Other embodiments of the invention will be described in followingdetailed description of embodiments, which is to be read in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system which comprises a clock recoverysystem according to an embodiment of the invention.

FIG. 2 is a high-level block diagram of a multilevel clock recoverysystem according to an embodiment of the invention.

FIG. 3A schematically illustrates a 4-level PAM scheme, as well asdecode functions of decode circuitry in the multilevel clock recoverysystem of FIG. 1, according to an embodiment of the invention.

FIG. 3B is an example illustration of a 4-level PAM signal which can bedecoded using a multilevel clock recovery system according to anembodiment of the invention.

FIG. 4 is a high-level conceptual illustration of a two-stage postcursorISI equalizer system according to an embodiment of the invention.

FIG. 5 is a block diagram of a parallel implementation of the two-stagepostcursor ISI equalizer system of FIG. 4, according to an embodiment ofthe invention.

FIG. 6 is a block diagram of a hardware realization of a multilevelclock recovery system according to an embodiment of the invention, whichis based on a parallel implementation of the high-level clock recoverysystem of FIG. 2.

DETAILED DESCRIPTION

Embodiments of the invention will now be discussed in further detailwith regard to digital receiver systems and clock recovery techniquesfor use in digital receiver systems. As discussed in further detailbelow, embodiments described herein include asynchronous baud-rate clockrecovery systems and methods for high data rate serial receivers, inparticular, digital receivers with multilevel line modulation.Techniques are provide to implement digital baud-rate clock recoverysystems in modern (nominal 14 nm) CMOS technology to supportindustry-evolving 56 Gb/s per lane data rates using 4-level linemodulation (e.g., 4-level PAM) while maintaining backward compatibilitywith 2-level NRZ systems.

FIG. 1 is a block diagram of a system which comprises a clock recoverysystem according to an embodiment of the invention. In particular, FIG.1 illustrates a system 100 comprising a data transmitter 110, acommunications channel 120, and a data receiver 130. The datatransmitter 110 comprises a multilevel pulse amplitude modulation (PAM)module 112. The data receiver 130 comprises an input amplifier 140, ananalog-to-digital converter (ADC) 150, a digital equalization system160, and a clock recovery system 170. Embodiments of the clock recoverysystem 170 will be discussed in further detail below with reference toFIGS. 2, 3A/3B, 4, 5, and 6, for example. The system 100 may beimplemented in various applications in which digital data streams aretransmitted between compute systems or components. For example, thesystem 100 may be implemented on a multi-chip module to enablechip-to-chip communication, wherein the data transmitter 110 is locatedon one chip, and the data receiver 130 is located on another chip, andthe communications channel 120 comprises a microstrip transmission lineformed on a package carrier, for example.

The data transmitter 110 transmits a high data rate bit stream to thedata receiver 130 over the communications channel 120. Specifically, inone embodiment of the invention, the multilevel PAM modulation module112 is configured to modulate a serial data stream to be transmittedover the communications channel 120 using n-level PAM, which results inthe generation of a symbol stream where each symbol has one of ndiscrete amplitudes (of some power of two). For example, in 4-level PAMthere are 2² possible discrete pulse amplitudes, in 8-level PAM thereare 2³ possible discrete pulse amplitudes, and in 16-level PAM there are2⁴ possible discrete pulse amplitudes, etc. For purposes ofillustration, embodiments of the invention will be discussed in thecontext of 2-level and 4-level PAM signals, although the techniquesdiscussed herein can be extended to multilevel PAM where n is greaterthan 4.

For example, FIG. 3A schematically illustrates a 4-level PAM scheme 300which can be implemented by the multilevel PAM module 112, and FIG. 3Billustrates an example 4-level PAM signal which can be generated by themultilevel PAM modulation module 112 a 4-level PAM scheme 300 based on agiven data stream shown in FIG. 3B. In particular, with a 4-level PAMscheme, each of two successive data bits in a given data bit stream areconverted to a single data symbol. With the 4-level PAM scheme 300 shownin FIGS. 3A and 3B, each data symbol is represented by one of 4 discretevoltage amplitudes, e.g., −1.5, −0.5, +0.5, +1.5, depending on thevalues of the two bits that are represented by the data symbol.Specifically, each data symbol represents two bits, comprising a MSB(most significant bit) and a LSB (least significant bit).

In the exemplary 4-level PAM scheme 300 shown in FIG. 3A, the MSB of agiven symbol is accorded either a nominal value of +1 when the MSB islogic “1” or a nominal value of −1 when the MSB is logic “0”. Moreover,the LSB of a given symbol is accorded either a nominal value of +0.5when the LSB is logic “1” or a nominal value of −0.5 when the LSB islogic “0”. The discrete PAM value of a given symbol is determined byadding the nominal values of the MSB and LSB. For example, based on thescheme shown in FIG. 3A, a data symbol represented by two data bits 00will have a pulse amplitude of −1.5 (i.e., −1+−0.5), and a data symbolrepresented by two data bits 01 will have a pulse amplitude of −0.5(i.e., −1+0.5). Further, a data symbol represented by two data bits 10will have a pulse amplitude of +0.5 (i.e., +1+−0.5), and a data symbolrepresented by two data bits 11 will have a pulse amplitude of +1.5(i.e., +1+0.5).

FIG. 3B illustrates an example 4-level PAM signal that is generatedbased on the above PAM scheme 300 for a data stream of 0001101101 over 5symbol periods. In a first symbol period T, a PAM signal value of −1.5represents the first two successive data bits 00. In a second symbolperiod 2T, a PAM signal value of −0.5 represents the next two successivedata bits 01. In a third symbol period 3T, a PAM signal value of +0.5represents the next two successive data bits 10. In a fourth symbolperiod 4T, a PAM signal value of +1.5 represents the next two successivedata bits 11. In a fifth symbol period 5T, a PAM signal value of −0.5represents the next two successive data bits 01. With 4 level PAMscheme, a data stream received at 56 Gb/s is actually transmitted overthe communications channel 120 at a corresponding BAUD rate of 28 Gb/sbecause each transmitted symbol represent two data bits.

The data receiver 130 receives a 4-level PAM signal that is transmittedover the communications channel 120. Because of the band-limitingcharacteristics of the communications channel 120 (and/or othertransmission characteristics), the transmitted PAM symbol stream can bedistorted (e.g., spreading of the symbol pulses) causing intersymbolinterference (ISI). The ISI is removed using techniques discussed below.The received analog signal is amplified by the input amplifier 140. Theamplified analog signal (which is output from the amplifier 140) isconverted to a digital format by the ADC 150. The ADC 150 uses asampling clock signal that is generated and output from the clockrecovery system 170 to sample the received analog signal. The digitalsignal output from the ADC 150 is processed by the digital equalizationsystem 160 and the clock recovery system 170 to generate estimates ofdata received from the data transmitter 110.

In particular, the digital equalization system 160 is configured tofilter the received data stream using a digital filter (e.g., FIR(finite impulse response) filter), which removes the ISI from thereceived symbol stream using known techniques. As is known in the art,an equalization process essentially inverts an impulse response of thechannel and generates an equalized impulse response having zero ISI andchannel distortion. Equalization methods are known in the art, thedetails of which are not needed for one of ordinary skill in the art tounderstand embodiments of the invention described herein.

The clock recovery system 170 is configured to extract a phase-accuratesample clock signal from the received data stream, wherein the sampleclock signal is used to clock the ADC 150 so that the ADC samples aretaken at an optimum sample time to optimize data decode. For example, asingle ADC sample is taken per each received 4-level symbol to minimizepower draw of the system. For maximum implementation efficiency, clockrecovery techniques discussed herein are configured to recover the ADCsample clock signal from the same digitized samples that are used torecover the data. A system which recovers timing information from thesame digitized samples used to recover data is referred to in thisdisclosure as a “baud-rate timing recovery system.” Such a system isdesired since it minimizes the sampling rate needed by the ADC to theminimum possible sample rate required to recover the data, and hencereduces system power and complexity.

One technique that is used for clock recovery in baud-rate sampledsystems is known as the Mueller-Muller based approach. The premisebehind this approach is to determine a data sample timing point wherethe ISI arising from a previous transmitted symbol (generatingpostcursor ISI at the data sample point) and a next transmitted symbol(generating precursor ISI at the data sample point) are balanced. In apractical realization of the Mueller-Muller approach, precursor symbolISI and a portion of postcursor symbol ISI are first removed by afeedforward equalizer (FFE). Next, a decision-feedback equalizer (DFE)is used to remove the remaining postcursor symbol ISI. Following removalof the ISI terms from the previous and next symbol, sign-error terms EAP(sign error at mean positive signal value AP) and EAN (sign error atmean negative signal value AN) at the data sample point are determined,which are utilized to determine a relative strength of any residualpostcursor and precursor ISI following the FFE and DFE equalization.These sign error terms EAP and EAN along with an estimate of the databit are sent to a phase-detector.

The phase detector determines an early-late timing adjust signal whichis sent to a loop filter, wherein the loop filter is implemented, forexample, as a proportional-integral (PI) filter. The loop filter in turndrives a phase adjustment system, commonly realized as a phase rotator,to adjust the sampling phase of the ADC sample clock to a point wherethe precursor and postcursor ISI at the data sample time are bothbalanced to zero (or to small residual values) after ISI equalization bythe FFE and DFE. This method of clock recovery is well known in the artand is most commonly employed in digital receiver systems since adigital receiver is most power efficient when it only needs to take oneADC sample per received symbol.

As data rates have evolved to 56 Gb/s in modern wireline communicationstandards, the line signaling has changed from 2 levels per symbol to 4levels per symbol, which complicates a practical hardware implementationof a baud-rate clock recovery system. For example, a 4-level DFEfunction is difficult to realize because there is too much delayinvolved in computing decision-feedback terms from the 4-level signal,so the DFE timing loop is difficult to close in modern CMOS logicrealizations. This problem can be solved by adding complex pipelining tothe DFE design, but this increases processing latency and powerconsumption. Processing latency in particular is not desired since ithas the effect of destabilizing the clock recovery loop which increasesjitter on the recovered clock and, in turn, can significantly degradereceiver bit-error-rate (BER).

Furthermore, the use of 4-level PAM signaling makes a straightforwardimplementation of a baud-rate phase detector more complex, since a +/−1BAUD ISI correlation must be performed against 4 data levels and 3 ISIthreshold combinations instead of two data levels and one ISI thresholdcombination as in a NRZ system. In this regard, embodiments of theinvention as discussed herein include systems and methods to addressthese issues, and provide for a practical realization of a power andarea efficient, low-latency baud-rate clock recovery system for use in,e.g., state of the art 56 Gb/s digital 4-level wireline data receiversystems, while maintaining support for 2-level operation in 28 Gb/s andlower NRZ systems.

FIG. 2 is a block diagram of a high data rate multilevel clock recoverysystem according to an embodiment of the invention. More specifically,FIG. 2 is a block diagram of an asynchronous baud-rate clock recoverysystem 200 according to an embodiment of the invention, which can beimplemented in either a 2-level or a 4-level high data rate receiversystem to achieve low processing latency and low implementationcomplexity. In one embodiment of the invention, the clock recoverysystem 200 is an extension of the Mueller-Muller based approachdiscussed above, wherein a novel DFE function (which is used to removeISI prior to a Mueller-Muller phase detector) is realized using atwo-stage postcursor ISI equalization system comprising a DFE stagewhich operates on a most significant component (e.g., MSB) of a given4-level PAM data symbol, followed by a DFFE (decision-feedforwardequalizer) stage which operates on a least significant component (e.g.,LSB) of the given 4-level PAM data symbol. As explained in furtherdetail blow, practical hardware implementations of a two-stagepostcursor ISI equalization system according to embodiments of theinvention allow for closing of the DFE feedback timing loop in high datarate applications (e.g., 56 Gb/s).

In addition, in parallel with the DFFE stage, an estimate of the leastsignificant component (e.g., LSB) of the given 4-level PAM data symbolis subtracted from an equalized 4-level PAM data symbol to convert the4-level PAM data symbol to a 2-level symbol which is subsequentlyprocessed with an efficient 2-level Mueller-Muller phase detector. Inthis regard, as explained in further detail below the clock recoverysystem 200 of FIG. 2 avoids the complexity issues associated with havingto implement a phase detection function in hardware to correlate +−1BAUD ISI against 4 levels, as opposed to two levels. For example, in oneembodiment of the invention, this complexity is avoided by implementinga speculative multiplexing scheme that is configured to simultaneouslysubtract a current LSB component from an equalized 4 level symbol at thesame time that the DFFE stage removes the postcursor ISI of theequalized 4-level symbol, which arises from a previous received LSBestimate.

Referring to FIG. 2, the digital baud-rate clock recovery system 200comprises ADC circuitry 202, FFE circuitry 204, equalizer tap valuedetermination logic 206, shift logic circuitry 208, MSB-DFE circuitry210, MSB decode circuitry 212, LSB decode circuitry 214, LSB-DFFE summercircuitry 216, multiplexer circuitry 218, sign error detection circuitry220, phase error detection circuitry 222, loop filter circuitry 224,phase adjustment circuitry 226, and control logic 228. In one embodimentof the invention, the equalizer tap value determination logic 206 andcontrol logic 228 comprises state machines (e.g., implemented infirmware), and the functions of the ADC circuitry 202, the FFE circuitry204, the shift logic circuitry 208, the MSB-DFE circuitry 210, the MSBdecode circuitry 212, the LSB decode circuitry 214, the LSB-DFFE summercircuitry 216, the multiplexer circuitry 218, the sign error detectioncircuitry 220, the phase error detection circuitry 222, the loop filtercircuitry 224, and the phase adjustment circuitry 226, are implementedin hardware.

In operation, the ADC circuitry 202 receives as input an analog signal(e.g., a 4-level PAM signal) and samples the analog signal via an ADCsample clock to generate a digital signal comprising a sequence ofdigital samples. The digital signal is input to the FFE circuitry 204and to the equalizer tap value determination logic 206. The equalizertap value determination logic 206 is configured to determineequalization coefficient values (“FFE tap values”) which are output tothe FFE circuitry 204. The FFE circuitry 204 is configured to utilizethe FFE tap values (e.g., postcursor, precursor taps) to cancelprecursor ISI of the digital sample and to partially cancel postcursorISI of the digital samples. Details of the FFE circuitry 204 accordingto an embodiment of the invention will be discussed in further detailbelow with reference to FIG. 6.

The equalizer tap value determination logic 206 is further configured todetermine a 1-tap postcursor DFE tap value (“H1”). The value H1 is inputto the control logic 228 to generate a coefficient MSBH1 which is inputto the MSB-DFE circuitry 210. The equalizer tap value determinationlogic 206 is configured as a state machine that implements any of avariety of methods commonly known in the art to generate and/ordynamically update the equalization coefficient values (e.g., FFE tapvalues and H1) for the FFE/DFE equalization functions, wherein thevalues are determined to minimize or eliminate the ISI contribution fromprevious and next data samples at a given data sampling point followingthe FFE/DFE equalization.

In one embodiment, the partially equalized digital samples that areoutput from the FFE circuitry 204 are input to the shift logic circuitry208. The shift logic circuitry 208 is configured to scale a level of thedigital samples in response to a control signal AGC SHIFT that isgenerated and output from the control logic 228, wherein the scalingserves to avoid digital underflow or overflow in remaining processingblocks.

Following the AGC shift, the partially equalized digital samples areinput to the MSB-DFE circuitry 210 which is configured to at leastpartially remove remaining postcursor ISI from the digital samples,which arises from the MSB components of previous data samples. TheMSB-DFE circuitry 210 performs MSB equalization using postcursor MSBH1tap values that are generated and output from the control logic 228. Inaddition, the MSB-DFE circuitry 210 operates in conjunction with the MSBdecode circuitry 212, wherein the MSB decode circuitry 212 is configuredto determine the sign (+ or −) of MSB components of the digital samples,wherein the MSB sign information is utilized by the MSB-DFE circuitry toperform the postcursor MSB ISI equalization.

The output of the MSB-DFE circuitry 210 is input to the LSB decodecircuitry 214 and to the LSB-DFFE summer circuitry 216. The LSB-DFFEsummer circuitry 216 comprises a first summer circuit 216-1, a secondsummer circuit 216-2, a third summer circuit 216-3, and a fourth summercircuit 216-4. The first summer circuit 216-1 adds the values −LSB and−LSBH1 to a digital sample output from the MSB-DFE circuitry 210. Thesecond summer circuit 216-2 adds the values −LSB and +LSBH1 to thedigital sample output from the MSB-DFE circuitry 210. The third summercircuit 216-3 adds the values +LSB and −LSBH1 to the digital sampleoutput from the MSB-DFE circuitry 210. The fourth summer circuit 216-4adds the values +LSB and +LSBH1 to the digital sample output from theMSB-DFE circuitry 210. The values LSB and LSBH1 are generated and outputfrom the control logic 228.

The LSB-DFFE summer circuitry 216 is configured to perform DFFEequalization of the partially equalized digital samples (which areoutput from the MSB-DFE circuitry 210) by removing postcursor ISI fromthe digital samples, which arises from the LSB components of previoussamples. The MSB-DFFE summer circuitry 216 performs LSB equalization byadding (or subtracting) a postcursor LSBH1 tap value from the partiallyequalized digital samples received from the MSB-DFE circuitry 210. Inaddition, the MSB-DFFE summer circuitry 216 either adds or subtracts anLSB value from the equalized digital samples received from the MSB-DFEcircuitry 210 to essentially remove the LSB component from the digitalssamples. The MSB-DFFE summer circuitry 216 operates in conjunction withthe LSB decode circuitry 214, wherein the LSB decode circuitry 214 isconfigured to determine the sign (+ or −) of LSB components of thedigital samples. The LSB sign information is utilized by the 4:1multiplexer circuitry 218 to select one of the summed outputs of thesummer circuits 216-1, 216-2, 216-3 and 216-4.

More specifically, in the embodiment shown in FIG. 6, the 4:1multiplexer circuitry 218 is responsive to a select control signal[DLSB(i): DLSB(i−1)] which is output from the LSB decode circuitry 214.The control signal DLSB(i) represents a decoded LSB of a current datasample (i), which will have a bit value of logic “1” or logic “0”depending on the whether the decoded LSB value of the current datasample (i) is positive or negative. Similarly, the control signalDLSB(i−1) represents a decoded LSB of a previous data sample (i−1),which will have a bit value of logic “1” or logic “0” depending on thewhether the decoded LSB value of the previous data sample (i−1) ispositive or negative.

The LSB sign information [DLSB(i): DLSB(i−1)] is input to select controlinputs of the 4:1 multiplexer circuitry 218 to select one of the summedoutputs of the summer circuits 216-1, 216-2, 216-3 and 216-4. Theselected outputs of the 4:1 multiplexer circuitry 218 comprise 2-levelequalized data samples which include only the MSB components (with LSBcomponents removed) and which have no or minimal postcursor MSB ISI andLSB ISI.

In one embodiment of the invention, the MSB-DFE circuitry 210, the MSBdecode circuitry 212, the LSB decode circuitry 214, the LSB-DFFE summercircuitry 216, and the 4:1 multiplexer circuitry 218 collectivelyimplement a two-stage postcursor ISI equalization system which isconfigured to efficiently emulate a 4-level DFE system, while convertingthe 4-level equalized signals to 2-level equalized signals. Morespecifically, the MSB-DFE circuitry 210 and the MSB decode circuitry 212collectively implement a DFE stage of a two-stage postcursor ISIequalization system, wherein the DFE stage operates on a mostsignificant component (e.g., MSB) of a given 4-level PAM data symbol tothereby remove postcursor ISI from the digital samples, which arisesfrom the MSB components of previous samples.

In addition, the LSB decode circuitry 214, the LSB-DFFE summer circuitry216, and the 4:1 multiplexer circuitry 218 collectively implement a DFFEstage of a two-stage postcursor ISI equalization system, wherein theDFFE stage operates on a least significant component (e.g., LSB) of thegiven 4-level PAM data symbol to thereby remove postcursor ISI from thedigital samples, which arises from the LSB components of previoussamples. In addition, DFFE stage is configured to convert a 4-levelsignal into a nominal 2-level signal by subtracting the LSB componentfrom a current 4 level symbol at the same time that the DFFE stage issubtracting the postcursor LSB ISI from the current 4-level symbol,which arises from the LSB component of a previous symbol.

The two-level output from the multiplexer circuity 218 is then processedby the sign error detection circuitry 220 to determine ISI error at thedata sample point. In particular, the sign error detection circuitry 220comprises a first summer circuit 220-1, a second summer circuit 220-2,sign determination circuitry 220-3, and 2:1 multiplexer circuitry 220-4.The first summer circuit 220-1 adds a value AP to the output of the 4:1multiplexer circuitry 218, while the second summer circuit 220-2 adds avalue AN to the output of the 4:1 multiplexer circuitry 218. The valuesof AP and AN are generated and output from the control logic 228.

The sign determination circuitry 220-3 is configured to determine thesign of the output signal of the 4:1 multiplexer circuitry 218, andoutputs a signal D which represents a determined sign of the currentdata sample (e.g., D is either a logic “1” or logic “0” depending onwhether the sign is positive or negative). The signal D is utilized as aselect control signal to the 2:1 multiplexer circuitry 220-4 to selectone of the outputs of the first and second summer circuits 220-1 and220-2, based on the determined sign. The output of the 2:1 multiplexercircuity 220-4 represents an error signal E.

The outputs E and D of the 2:1 multiplexer circuitry 220-4 and the signdetermination circuitry 220-3, respectively, are input to the phaseerror detection circuitry 222. The phase error detection circuitry 222is configured to determine an early/late (E/L) phase correction signalto drive the loop filter circuitry 224. The output of the loop filtercircuitry 224 is input to the phase adjustment circuitry 226 whichcloses the clock recovery loop to drive the ADC sample clock for the ADCcircuitry 202.

As shown in FIG. 2, the control logic 228 receives various signals E, Dand H1, which are generated by components of the clock recovery system200, as well as other control signals such as TIMING GAIN, TIMING OFFSETand OPERATING MODE, which are generated by other control logic of thehost system (e.g., digital receiver system). The control logic 228generates and outputs various signals including AP, AN, MSBH1, LSBH1,LSB, and AGC SHIFT, which are utilized by various components of theclock recovery system 200, as discussed herein. Details of the how thecontrol logic 228 utilizes the various input control signals andgenerates the various output control signals will be discussed infurther detail below.

FIG. 3A schematically illustrates a 4-level PAM scheme 300 (as discussedabove) as well as decode functions 310 and 320 of the MSB decodecircuitry 212 and the LSB decode circuitry 214, respectively, accordingto an embodiment of the invention. In particular, as discussed above,FIG. 3A illustrates a 4-level PAM scheme 300 in which a first component(e.g., MSB data value) of a given data symbol modulates a sign of anormalized +/−1 level symbol, while a second component (e.g., LSB datavalue) of the given data symbol modulates the sign of a normalized+/−0.5 level symbol. The sum of these two components creates the desired4-level data symbol which encodes two components (e.g., MSB and LSB) ofinformation.

As further shown in FIG. 3A, the decode function 310 of the MSB decodecircuitry 212 is configured to determine a value of a digital sampleoutput from the MSB-DFE circuitry 210 and output a logic “1” if thevalue of the digital sample (s) is ≧0, and output a logic “0” otherwise.Moreover, the decode function 320 of the LSB decode circuitry 214 isconfigured to determine the normalized value of the data sample outputfrom the MSB-DFE circuitry 210 and output one of the following binaryvalues as a function of the normalized value of the data sample, asfurther shown on the following table:

TABLE 1 Normalized Signal Value Output From MSB-DFE Circuit Output ofLSB Decode Circuit ≧1 1 ≧0, <1 0 >−1, <0 1 ≦−1 0

FIG. 4 is a high-level conceptual illustration of a two-stage postcursorISI equalizer system 400 according to an embodiment of the invention,which can be implemented in the clock recovery system 200 of FIG. 2. Thetwo-stage postcursor ISI equalizer system 400 comprises a MSB-DFE stage410 and a LSB-DFFE stage 420. The MSB-DFE stage 410 comprises summercircuitry 412, MSB decode circuitry 414, delay circuitry 416, andmultiplication circuitry 418. The LSB-DFFE stage 420 comprises summercircuitry 422, LSB decode circuitry 424, delay circuitry 426, andmultiplication circuitry 428.

The MSB-DFE stage 410 is configured to remove postcursor ISI of a givendata sample which arises from the MSB component of a previous datasample. In the feedback path of the MSB-DFE stage 410, the MSB decodecircuitry 414 determines the sign of the feedback term based solely onthe sign of the output sample at the output of the summer circuitry 412.This avoids the need for implementing a time consuming digital thresholdcomparison, since the sign of the output sample may be determineddirectly by the MSB of the output sample of the MSB-DFE stage 410. Forexample, as noted above with reference to FIG. 3A, the MSB decodefunction 310 outputs a logic “1” if the digital sample is ≧0, andoutputs a logic “0” otherwise. The delay circuitry 416 is configured toprovide a one sample delay in the feedback path. The multiplicationcircuitry 418 is configured to apply a MSBH1 equalization tap value tothe output sample in the feedback path, and the equalized output samplein the feedback path is added to a next sample via the summer circuity412.

The LSB-DFFE stage 420 is configured to remove postcursor ISI of thegiven data sample which arises from the LSB component of the previousdata sample. In particular, at the output of the MSB-DFE stage 410,there still remains undesired postcursor ISI on the equalized datasample, which arises from the LSB component of the previous data sample.This LSB ISI is removed by the LSB-DFFE stage 420 using adecision-feedforward equalization. In a feedforward path, the LSB decodecircuitry 424 estimates the sign of the LSB at the output of the MSB-DFEstage 410 using the LSB decode function 320 as discussed above withreference to FIG. 3A, and as illustrated in TABLE 1 above. The delaycircuitry 426 is configured to provide a one sample delay in thefeedforward path. The multiplication circuitry 428 is configured toapply an equalization tap value LSBH1 (which comprises a fraction of H1,for example) to the output sample in the feedforward path, and theequalized output sample in the feedforward path is added to a nextsample via the summer circuity 422.

FIG. 5 is a block diagram of a parallel implementation of the two-stagepostcursor ISI equalizer system of FIG. 4, according to an embodiment ofthe invention. More specifically, FIG. 5 schematically illustrates atwo-stage postcursor ISI equalizer system 500 comprising a MSB-DFE stage510 and an LSB-DFFE stage 520. The two-stage postcursor ISI equalizersystem 500 comprises a parallel configuration in which blocks of Nsuccessive samples (i), (i+1), (i+(N−1)), are clocked in parallel into Nparallel inputs to the MSB-DFE stage 510, and processed in N parallelprocessing paths P(i), P(i−1), P(i+(N−1)) of the two-stage postcursorISI equalizer system 500. In one embodiment of the invention, N=8.

In the parallel embodiment shown in FIG. 5, each processing path P(i),P(i−1), P(i+(N−1)) of the MSB-DFE stage 510 comprises a summer circuit512, and a feedback path having MSB decode circuitry 514 andmultiplication circuitry 518, which have similar functions as the decodecircuitry 414 and multiplication circuitry 418 discussed above withreference to FIG. 4, for example. The feedback path of a givenprocessing path of the MSB-DFE stage 510 is coupled between the outputof the summer circuity 512 in the given processing path and the input ofthe summer circuitry 512 in a next processing path. For example, asshown in FIG. 5, the output of the summer circuit 512 in the processingpath P(i) is equalized in the feedback path and input to the summercircuitry 512 in the next processing path P(i+1), wherein the equalizedsample of the previous sample (i) is added with the current sample(i+1).

As further shown in FIG. 5, the output of the summer circuit 512 in theprocessing path P(i+(N−1)) is equalized in the feedback path of theprocessing path P(i+(N−1)) and input to the summer circuitry 512 in thenext processing path P(i), wherein the equalized sample of the previoussample P(i+(N−1)) is added to the current sample (i). The feedback pathof the processing path P(i+(N−1)) comprises a delay circuit 516 toprovide at least one sample delay to provide time to clock in the nextblock of N successive data samples, wherein the new sample (i) of thenext block of N successive data samples would be the next successivesample of the sample (i+(N−1)) of the current block of N successive datasamples.

Each processing path P(i), P(i−1), P(i+(N−1)) comprises a delay circuit530 to couple the corresponding processing paths of the MSB-DFE stage510 and the LSB-DFFE stage 520. The delay circuits 530 are configured toprovide at least one sample delay. In the parallel embodiment shown inFIG. 5, each processing path P(i), P(i−1), P(i+(N−1)) of the MSB-DFFEstage 520 comprises a summer circuit 522, and a feedforward path havingLSB decode circuitry 524 and multiplication circuitry 528, which havesimilar functions as the decode circuitry 424 and multiplicationcircuitry 428 discussed above with reference to FIG. 4, for example.

The feedforward path of a given processing path of the LSB-DFFE stage520 is coupled between the corresponding processing path output of theMSB-DFE stage 510 and the input of the summer circuitry 522 in a nextprocessing path of the LSB-DFFE stage 520. For example, as shown in FIG.5, the feedforward path of the processing path P(i) of the LSB-DFFEstage 520 is coupled between the corresponding processing path P(i)output of the MSB-DFE stage 510 and the input of the summer circuitry522 in a next processing path P(i+1) of the LSB-DFFE stage 520. In eachfeedforward path of the LSB-DFFE stage 520, a previous sample (e.g., MSBequalized sample (i)) is equalized and input to the summer circuit 522of the next processing path, wherein it is added to the current sample(e.g., MSB equalized sample (i+1)).

As further shown in FIG. 5, the output of the feedforward path of theprocessing path P(i+(N−1)) of the LSB-DFFE stage 520 is coupled to aninput of the summer circuit 522 in the processing path P(i) of theLSB-DFFE stage 520, wherein a previous equalized data sample output fromthe feedforward path of the processing path P(i+(N−1)) is added to thecurrent sample (e.g., MSB equalized sample (i)) in the processing pathP(i). The feedforward path of the path P(i+(N−1)) of the LSB-DFFE stage520 comprises a delay circuit 526 to provide at least one sample delayto provide time to clock in the next block of N successive data samples,as discussed above.

In the parallel embodiment shown in FIG. 5, the MSB-DFE output (i+(N−1))will not be known until the MSB decode and DFE summer operations ripplethrough all previous N−1 parallel processing paths. As such, it isdesirable to minimize the time that it takes to perform the MSB decodeand DFE summation operations to enable an efficient logic realizationthat meets timing requirements. At the very least, the parallelimplementation in FIG. 5 slows down the clock rate of the parallelprocessing paths of the MSB-DFE stage 510 to a range of <4 GHz. Theslower clock rate of the parallel processing paths enables an efficientlogic realization of the MSB-DFE stage 510 to meet timing requirementsin modern CMOS technology digital logic (e.g., to close the MSB-DFEfeedback timing loop). By way of example, for a 56 Gb/s 4-level PAMdigital receiver, wherein the PAM symbols are transmitted at a rate of28 G symbols per second, assuming that the number N of parallelprocessing paths is 8, then the clock rate (C8 clock rate) in each ofthe parallel paths is 28/8=3.5 G samples per second. A practicalhardware implementation of a parallel MSB-DFE stage 510 according to anembodiment of the invention will be discussed below with reference toFIG. 6.

The use of a two-stage postcursor ISI equalization system of FIG. 5, forexample, is advantageous for various reasons as discussed herein. Forexample, because the LSB-DFFE stage 520 utilizes feedforward ISIcancellation instead of feedback cancellation, there is no length-Nfeedback ripple loop to settle, As such, the extra time that is neededto perform the LSB decode functions by the LSB decode circuitry 524 withrequired digital level comparisons (equivalent to adds/subtracts) can bereadily accommodated in a practical hardware realization in modern CMOStechnology at “C8” clock data rates (i.e. in a range of less than 4 GHzlogic clock).

It is to be noted that because the data samples at the path outputs ofthe of MSB-DFE stage 510 may still contain undesired postcursor ISIarising from the uncorrected LSB ISI, the combination of thisuncorrected ISI with other uncompensated ISI terms and noise mayincrease the probability of incorrectly determining the LSB value for areceived signal. However, in a practical embodiment, the LSBdetermination errors can be tolerated in the clock recovery systembecause of a large amount of filtering that is performed by the loopfilter circuitry 224 (FIG. 2). In particular, in one embodiment of theinvention, the loop filter 224 is implemented as a proportional-integralfilter, wherein the proportional and integral gain is configured toachieve a control loop bandwidth in the range of about BAUD/2000 orlower, where BAUD is the line symbol transmission rate in symbols/s. Asa result, thousands of samples are averaged within the loop bandwidth ofthe clock recovery control system. This large amount of averagingincreases the tolerance of the clock recovery system to intermittent MSBand LSB decision errors arising from channel noise, crosstalk, anduncompensated ISI.

FIG. 6 is a block diagram of a hardware realization of a multilevelclock recovery system 600 according to an embodiment of the invention,which is based on a parallel implementation of the high-level clockrecovery system of FIG. 2. For ease of illustration, FIG. 6schematically depicts the circuitry along one processing path P(i) of aN parallel processing paths of the multilevel clock recovery system 600(e.g., N=8). The circuitry shown in FIG. 6 comprises a portion of eachof the ADC circuity 202, the FFE circuitry 204, the shift logiccircuitry 208, the MSB-DFE circuitry 210, the MSB decode circuitry 212,the LSB decode circuitry 214, the LSB-DFFE summer circuitry 216, the 4:1multiplexer circuitry 218, the sign error detection circuitry 220, andthe phase error detection circuitry 222 of FIG. 2. While FIG. 6 showsone processing path of N parallel processing paths of the clock recoverysystem 600, it is to be understood that each of the N processing pathsis implemented using the hardware configuration as shown in FIG. 6.

In this regard, it is to be understood that in the example embodiment ofFIG. 6, the index (i) denotes the index for a current sample (i) on agiven processing path P(i), wherein the processing path P(i) shown inFIG. 6 represents any one of the N parallel processing paths (e.g.,processing paths P(i), P(i−1), P(i+(N−1)), as shown in FIG. 5). Theindex (i−1) denotes a previous data sample or previous processing path(relative to the index (i)), and the index (i+1) denotes a next datasample or a next processing path (relative to the index (i)).

Referring to FIG. 6, in one embodiment of the invention, each processingpath P(i) receives as input three ADC samples, including, a currentsample ADC(i), a previous sample ADC(i−1) and a next sample ADC(i+1).The ADC samples ADC(i−1), ADC(i) and ADC(i+1) are temporarily held incorresponding latches 603. In particular, the latches 603 comprise afirst latch 603-1 to store the previous sample ADC(i−1), a second latch603-2 to store a current sample ADC(i), and a third latch 603-3 to storea next digitized sample ADC(i+1). The latches 603 are controlled by aclock signal C8 which is a fraction (e.g., ⅛) of the frequency of theADC sampling clock.

FIG. 6 illustrates a practical hardware implementation the FFE circuitry204 in the given processing path. In one embodiment, the FFE circuitry204 comprises an n-tap FFE circuit (e.g., n=3) comprising a firstmultiplier 604-1, a second multiplier 604-2, and a summer circuit 604-3.The first multiplier 604-1 is configured to multiply a value of theprevious sample ADC(i−1), which is output from the latch 603-1, by a FFEpostcursor value. The second multiplier 604-2 is configured to multiplya value of the next digitized sample ADC(i+1), which is output from thelatch 603-3, by a FFE precursor value. The summer circuit 604-3 isconfigured to add a value of the current sample ADC(i) to the correctedvalues of the previous sample ADC(i−1) and the next sample ADC(i+1)output from the respective multipliers 604-1 and 604-2. The output ofthe summer circuit 604-3 is a FFE equalized version of the currentdigitized sample ADC(i).

In the embodiment of FIG. 6, the FFE circuitry 204 comprises a minimum 3taps, one tap to cancel precursor ISI of the current sample ADC(i) andone tap to partially cancel postcursor ISI of the current sample ADC(i).In other embodiments, an n-tap FFE can be implemented with n>3. In apreferred practical embodiment, a 3 tap FFE is implemented to minimizeprocessing latency and hardware complexity of the FFE circuitry 204. Asnoted above, the FFE postcursor and precursor tap values are generatedby the equalizer tap value determination logic 206 (FIG. 2).

The clock recovery system 600 further comprises, in the given processingpath, a shift logic circuit 608, which is part of the shift logiccircuitry 208 of the clock recovery system of FIG. 2, and a latch 609connected to the output of the shift-left logic circuit 608. The latch609 is controlled by the same clock signal C8 which controls the latches603.

The embodiment of FIG. 6 further illustrates a practical hardwareimplementation of the MSB-DFE circuitry 210 (FIG. 2) using speculativeDFE feedback. The use of direct summer circuits (e.g., the summercircuits 512 of the MSB-DFE stage 510 shown in FIG. 5) may not bepractical due to excess logic delay for the summation function, whichcould prevent the clock recovery system from meeting logic timingconstraints. To address this issue, an efficient DFE summation functionaccording to one embodiment of the invention is realized usingspeculative computation of both possible DFE feedback states, one ofwhich being selected using a multiplexer based on control signals outputfrom the MSB decode circuitry 212.

In particular, the MSB-DFE circuitry 210 in the given processing pathshown in FIG. 6 comprises a first summer circuit 610-1, a second summercircuit 610-2, and a 2:1 multiplexer circuit 610-3. The MSB decodecircuitry 212 in the given processing path shown in FIG. 6 comprises MSBdecode circuits 612-1 and 612-2. As noted above, the MSB-DFE and MSBdecode circuitry 210/210 collectively provides a DFE stage of a twostage postcursor ISI equalization system according to an embodiment ofthe invention. The first summer circuit 610-1 adds a value −MSBH1 to anequalized digital sample that is output from the latch 609, and thesecond summer circuit 610-2 adds a value +MSBH1 to the equalized digitalsample that is output from latch 609. The 2:1 multiplexer circuit 610-3selectively outputs an MSB equalized output from one of the summercircuits 610-1 and 610-2 in response to a control signal output from theMSB decode circuit 612-1 (which is actually part of a previousprocessing path P(i−1)).

The MSB equalized output from the 2:1 multiplexer circuit 610-3 is inputto the MSB decode circuit 612-2 of the given processing path P(i). Inresponse, the MSB decode circuit 612-2 outputs a control signal to a 2:1multiplexer circuit of an MSB-DFE circuitry in the next processing pathP(i+1). The MSB decode circuits 612-1 and 612-2 are similar in functionto the MSB decode circuits 514 in different processing paths, as shownin FIG. 5. The hardware implementation of the MSB-DFE circuitry 210shown in FIG. 6 greatly speeds up the settling of the MSB-DFE feedbackloops since the use of the speculative 2:1 multiplexer circuit 610-3 issignificantly faster than implementing an add/subtract operation inhardware.

The output of the 2:1 multiplexer circuit 610-3 is also stored in alatch 613, which is controlled by the same clock signal C8 as theprevious latches 603 and 609. The output of the latch 613 is input tothe DFFE stage 214/216/218 which, in the given processing path, includesan LSB decode circuit 614, LSB-DFFE summer circuitry comprising a firstsummer circuit 616-1, a second summer circuit 616-2, a third summercircuit 616-3, and a fourth summer circuit 616-4, and a 4:1 multiplexercircuit 618.

The first summer circuit 616-1 adds the values −LSB and −LSBH1 to anMSB-DFE equalized digital sample output from the latch 613. The secondsummer circuit 616-2 adds the values −LSB and +LSBH1 to the MSB-DFEequalized digital sample output from the latch 613. The third summercircuit 616-3 adds the values +LSB and −LSBH1 to the MSB-DFE equalizeddigital sample output from the latch 613. The fourth summer circuit616-4 adds the values +LSB and +LSBH1 to the MSB-DFE equalized digitalsample output from the latch 613. The values LSB and LSBH1 areprecomputed by the control logic 228 (FIG. 2). The 4:1 multiplexer 618selectively outputs the MSB-DFE equalized digital sample (output fromthe MSB-DFE circuitry 201/212) summed to one of the pre-computedcombinations of the LSB and LSBH1 values, based on the values of theselect control signals S0 and S1.

In particular, in one embodiment of the invention, the 4:1 multiplexer618 is controlled by two select signals S0 and S1, wherein Si is a mostsignificant multiplexer select signal driving the 4:1 multiplexer 618,and S0 is a least significant multiplexer select signal driving the 4:1multiplexer 618. The select control signal S1 is driven by the output ofthe LSB decode circuit 614 for the current symbol of the currentprocessing path (i), whereas the select control signal S0 is driven byan output of a LSB decode circuit of a previous symbol of the previousprocessing path (i−1). As further shown in FIG. 6, the output of the LSBdecode circuit 614 drives a select control signal S0 input of a 4:1multiplexer circuit in a next processing path (i+1) for the next symbol(i+1).

In the embodiment of FIG. 6, the LSB decode circuit 614 receives theMSB-DFE equalized data sample that is output from the latch 613 in thecurrent processing path P(i) and determines a sign of the LSB componentof the equalized data sample. The LSB decode circuit 614 outputs a logic“0” (for negative sign) or logic “1” (for positive sign) depending onthe determined sign of the LSB component (e.g., see TABLE 1). The outputof the LSB decode circuit 614 serves as an S1 control signal for the 4:1multiplexer circuit 618 in the given processing path P(i), as well as anS0 control signal for the 4:1 multiplexer circuit in a next processingpath (i+1) for the next data sample (i+1). Furthermore, an LSB decodecircuit in the previous processing path P(i−1) determines a sign of theLSB component of the previous data sample (i−1) and outputs a logic “0”(for negative sign) or logic “1” (for positive sign) as an S0 controlsignal for the multiplexer circuit 618 in the current processing pathP(i).

As discussed above with reference to FIG. 2, the implementation of the4:1 multiplexer 618 serves to simultaneously remove both the postcursorLSB ISI and the LSB component from the current data sample (i). Inparticular, when the control signals S0/S1 are 1/1, the sum value at theoutput of the first summer 616-1 is selected by the 4:1 multiplexer 618.This is because a S0/S1 control signal of 1/1 indicates that the sign ofthe LSB component of the previous data sample (i−1) is positive, andthat the sign of the LSB component of the current data sample (i) ispositive. As such, the values −LSB and -LSBH1 are added to the currentdata sample (i) to remove the remove both the postcursor LSB ISI and theLSB component from the current data sample (i).

Further, when the control signals S0/S1 are I/O, the sum value at theoutput of the second summer 616-2 is selected by the 4:1 multiplexer618. This is because a S0/S1 control signal of I/O indicates that thesign of the LSB component of the previous data sample (i−1) is positive,and that the sign of the LSB component of the current data sample (i) isnegative. As such, the values −LSB and +LSBH1 are added to the currentdata sample (i) to remove the remove both the postcursor LSB ISI and theLSB component from the current data sample (i).

In addition, when the control signals S0/S1 are 0/1, the sum value atthe output of the third summer 616-3 is selected by the 4:1 multiplexer618. This is because a S0/S1 control signal of 0/1 indicates that thesign of the LSB component of the previous data sample (i−1) is negative,and that the sign of the LSB component of the current data sample (i) ispositive. As such, the values +LSB and −LSBH1 are added to the currentdata sample (i) to remove the remove both the postcursor LSB ISI and theLSB component from the current data sample (i).

Moreover, when the control signals S0/S1 are 0/0, the sum value at theoutput of the fourth summer 616-4 is selected by the 4:1 multiplexer618. This is because a S0/S1 control signal of 0/0 indicates that thesign of the LSB component of the previous data sample (i−1) is negative,and that the sign of the LSB component of the current data sample (i) isnegative. As such, the values +LSB and +LSBH1 are added to the currentdata sample (i) to remove the remove both the postcursor LSB ISI and theLSB component from the current data sample (i).

In this manner, the output of the 4:1 multiplexer circuit 618 comprisesan equalized 2-level signal which contains only the MSB component of thecurrent data sample (i), which is either a +1 or −1 normalized NRZvalue. As noted above, it is desirable to process a 2 level signal(output from the multiplexer circuit 618) using a baud-rate phasedetector, as compared to processing a 4-level signal using a baud-ratephase detector. This is because processing of a 4-level signal wouldrequire correlating four different data levels against ISI arising atthree threshold levels at the data sample time (as compared to twodata/one ISI threshold levels in a NRZ system), which would greatlyincrease the logic complexity of the system over a NRZ baud-rate phasedetector. Thus, the techniques described herein avoid this complexity bysubtracting the LSB component from the 4-level signal to produce anoutput that includes the MSB component of the signal. This allows use ofan efficient 2-level NRZ baud-rate phase detector to determineearly/late timing information, as explained in further detail below.

The sign error detection circuitry 220 in the given processing path P(i)comprises a first summer circuit 620-1, a second summer circuit 620-2,sign determination circuits 620-3, 620-4, 620-5, inverters 620-6, 620-7,620-8, and a 2:1 multiplexer circuit 620-9. The equalized 2-level signal(which is output from the 4:1 multiplexer circuit 618) is input to thefirst and second summer circuits 620-1 and 620-2, and the signdetermination circuit 620-5. The first summer circuit 620-1 subtracts amean positive value AP from the value of the equalized 2-level signaland outputs an error term EAP. The second summer circuit 620-2 subtractsa mean negative value AN from the value of the equalized 2-level signaland outputs an error term EAN. The error term EAP represents a signerror at the mean positive signal value AP, and the error term EANrepresents the sign error at the mean negative signal value AN.

The sign determination circuit 620-3 determines a sign of the EAP term,the sign determination circuit determines a sign of the EAN term, andthe sign determination circuit 620-5 determines a sign of the equalized2-level signal output from the 4:1 multiplexer 618. In one embodiment ofthe invention, the inverter circuits 620-6, 620-7, and 620-8 areimplemented to invert the outputs of the respective sign determinationcircuits 620-3, 620-4, and 620-5, so that if a given sign is determinedto be positive (which is a logic “0” most-significant bit value intwos-complement representation), a logic “1” value at the output of theinverter circuits 620-6, 620-7, and 620-8 will represent a positivesign, whereas a logic “0” value at the output of the inverter circuits620-6, 620-7, and 620-8 will represent a negative sign.

If the sign of the equalized 2-level signal is determined by the signdetermination circuit 620-5 to be positive, a D(i) value of 1 is outputfrom the inverter circuit 620-8. On the other hand, if the sign of theequalized 2-level signal is determined by the sign determination circuit620-5 to be negative, a D(i) value of 0 is output from the invertercircuit 620-8. The D(i) value is utilized as a select control signalthat is input to the 2:1 multiplexer circuit 620-9 to select either thesign bit of EAP or the sign bit of EAN as an error signal output E(i).For example, if the D(i) value is logic “1” (indicating that the sign ofthe equalized 2-level signal is positive), then the sign bit of EAP isselectively output by the 2:1 multiplexer circuit 620-9 as the errorsignal E(i). On the other hand, if the D(i) value is logic “0”(indicating that the sign of the equalized 2-level signal is negative),then the sign bit of EAN is selectively output by the 2:1 multiplexercircuit 620-9 as the error signal E(i).

The phase detector circuit 222 in the given processing path P(i)comprises a first XOR (exclusive OR) gate 622-1, a second XOR gate622-2, a first AND gate 622-3, and a second AND gate 622-4. The firstand second XOR gates 622-1 and 622-2 are configured as a phase detector,and the first and second AND gates are configured to generate acomplementary Early/Late control signal (EL1/EL0), which are well knowncircuit configurations for implementing a phase detector circuit. Asshown in FIG. 6, the first XOR gate 622-1 receives as input an errorsignal E(i) for the current data sample (i) and a value D(i−1) of theequalized 2-level signal of the previous processing path P(i−1). Thesecond XOR gate 622-2 receives as input an error signal E(i−1) for theprevious data sample (i−1) and the value D(i) of the current sample (i)(equalized 2-level signal) of the current processing path P(i).

The D(i) and E(i) bit sequences that are generated in the currentprocessing path P(i) are processed in combination with the D(i−1) andE(i−1) bit sequences that are generated in the previous processing pathP(i−1) to produce a digital Early/Late output signal (EL1, EL0) whichdrives the digital loop filter 224, as discussed above.

The combination of the bit sequences D(i) and E(i) is performed in amanner to balance the ISI correlation of precursor and postcursor ISI.The bit value D(i−1) imparts postcursor ISI on the current value D(i) sothe correlation of data with postcursor ISI is computed by D(i−1) XORE(i). The value D(i) imparts precursor ISI on value D(i−1) so precursorISI is computed by D(i) XOR E(i−1). The following truth tables show howan Early/Late timing signal is derived to drive a loop filter.

TABLE 2 Phase detection from D(i) and E(i) Sequences D(i) E(i − 1) ISID(i) XOR E(i − 1) Timing Sign 0 0 Precursor ISI+ 0 Early 0 1 PrecursorISI− 1 Late 1 0 Precursor ISI− 1 Late 1 1 Precursor ISI+ 0 Early D(i− 1) E(i) ISI D(i − 1) XOR E(i) Timing Sign 0 0 Postcursor ISI+ 0 Late 01 Postcursor ISI− 1 Early 1 0 Postcursor ISI− 1 Early 1 1 PostcursorISI+ 0 Late

TABLE 3 Early/Late Determination from Phase Detector EL(i) = D(i) XORE(i − 1) − D(i) XOR E(i − 1) D(i − 1) XOR E(i) D(i − 1) XOR E(i) 0(Early) 0 (Late) 0 (Ontime) 0 (Early) 1 (Early) −1 (Early) 1 (Late) 0(Late) 1 (Late) 1 (Late) 1 (Early) 0 (Ontime)

The Early/Late digital inputs to the loop filter 224 can be encoded inany desired manner depending on the hardware implementation. Forexample, in the block diagram in FIG. 6, the following “one hot”early/late encoding may be implemented as follows:

TABLE 4 Early/Late Encoding D(i) XOR E(i − 1) D(i − 1) XOR E(i) EL1(i)EL0(i) 0 (Early) 0 (Late) 0 0 0 (Early) 1 (Early) 1 0 1 (Late) 0 (Late)0 1 1 (Late) 1 (Early) 0 0

As noted above, the mean AP and AN values are computed by the controllogic 228 using the sign error E generated by the sign errordetermination circuitry 220 and the estimated data value D. In oneembodiment of the invention, the control logic 228 utilizes first-orderintegration loops to compute the AP and AN values based on the E and Dvalues. In particular, if the estimated data value D is positive,control logic 228 integrates the mean AP value toward a larger value ifthe error term EAP indicates a positive sign (meaning the receivedsignal is greater than AP), and integrates the mean AP value to asmaller value otherwise. If the estimated D value is negative, the meanvalue AN is updated in a corresponding manner. Once the first-orderintegration loops converge, the AP and AN values will represent the meanpositive and negative values of the samples at the input to the signerror determination circuitry 220.

To avoid overflow or underflow in the signal processing paths, thecontrol logic 228 will examines the values of AP and AN, and if they aretoo small, the control logic 228 will configure the AGC Shift function228 on the output of the FFE 204 to shift-left the values in the digitalregister a desired amount (effectively making the values larger in thefixed precision available in an implementation). The values AP and ANmay also become too large if the mean signal level output from the FFEcircuitry 204 is later increased for any reason after an AGC shift hasbeen applied. In this instance, the control logic 228 will reduce theamount of shift-left, which is being applied to the FFE output samples,down to a minimum value of 0.

In addition, in one embodiment of the invention, the control logic 228utilizes the inputs H1, TIMING GAIN, and TIMING OFFSET signals tocompute the output values MSBH1, LSB, LSBH1. The value H1 is thenormalized DFE postcursor ISI equalization value “H1” that is outputfrom the equalizer tap value determination logic 206. The TIMING GAINfactor is a control signal that is input to the control logic 228, whichin a preferred embodiment is typically set to a constant value of 0.85,but may be varied by an external process control if desired over atypical range from 0.7 to 0.9 to adjust a timing lock point on thereceived signal. The TIMING OFFSET factor is a control signal that isinput to the control logic 228, which is set to a nominal constant valueof 0, but may also be varied by an external control process if desiredto further adjust the timing lock point on the received signal. In atypical application, the TIMING GAIN is left at a nominal operationvalue of 0.85, while the TIMING OFFSET value may be increased from 0 toadjust the timing point earlier in time (toward a larger H1 ISI in thereceived signal) and decreased from 0 to adjust the timing point laterin time (toward a smaller H1 ISI in the received signal). Thisadjustment may be desired if it is determined (via an external controlprocess) that the system operating BER can be improved by advancing orretarding the timing lock phase of the clock recovery system.

In one embodiment of the invention, when the digital receiver isconfigured by the control input OPERATING MODE to function in 4-levelmode, the output values MSBH1, LSB, LSBH1 are computed as follows:MSBH1=(H1+Timing Offset)*(AP+AN)*0.5*Timing GainLSB=(AP+AN)*0.5/2LSBH1=H1*LSB

When the control signal OPERATING MODE input to the control logic 228specifies a 2-level operating mode, the control logic 228 computes theMSBH1 value the using the same formula above, but sets both the LSB andLSBH1 outputs to a value of 0.

Although embodiments have been described herein with reference to theaccompanying drawings for purposes of illustration, it is to beunderstood that the present invention is not limited to those preciseembodiments, and that various other changes and modifications may beaffected herein by one skilled in the art without departing from thescope of the invention.

We claim:
 1. A clock recovery system, comprising: an ADC(analog-to-digital converter) circuit configured to sample an analogsignal in response to sample a clock signal and generate a digitalsignal comprising sequence of digital data samples, wherein the analogsignal comprises a stream of data symbols, wherein each data symbolcomprises a plurality n of symbol components, including at least a firstsymbol component and a second symbol component, and wherein each digitaldata sample corresponds to one data symbol in the analog signal; a FFE(feed forward equalizer) circuit configured to remove precursor ISI(intersymbol interference) from the digital data samples and at leastpartially remove postcursor ISI from the digital data samples, andoutput FFE equalized data samples; a DFE (decision feedback equalizer)circuit configured to remove a first component of postcursor ISI fromthe FFE equalized data samples, and output DFE equalized data samples,wherein the first component of postcursor ISI of a given data samplearises from the first symbol component of a previous data sample; and aDFFE (decision feedforward equalizer) circuit configured to remove asecond component of postcursor ISI from the DFE equalized data samples,and generate DFFE equalized data samples, wherein the second componentof postcursor ISI of a given data sample arises from the second symbolcomponent of a previous data sample; wherein the DFFE circuit is furtherconfigured to remove all but one of the n symbol components from eachDFFE equalized data sample, and output an equalized 2-level digitalsignal comprising a sequence of equalized 2-level data samples; andwherein the analog signal comprises a 4-level PAM (pulse amplitudemodulation) signal, wherein each data symbol comprises amost-significant bit (MSB) as the first symbol component, and a leastsignificant bit (LSB) as the second symbol component, and wherein theequalized 2-level digital signal output from the DFFE circuit comprisesonly the MSB components of the digital data samples.
 2. The clockrecovery system of claim 1, wherein the DFE circuit comprises: a firstsummer circuit configured to add a postcursor correction value to acurrent FFE equalized data sample; a second summer circuit configured tosubtract the postcursor correction value from the current FFE equalizeddata sample; a multiplexer circuit connected to an output of each of thefirst and second summer circuits; and first component decode circuitconfigured to determine a sign of the first symbol component of aprevious FFE equalized data sample, and output a control signal thatindicates the determined sign of the first symbol component of aprevious FFE equalized data sample; wherein the control signal outputfrom the first component decode circuit controls the multiplexer circuitto select one of the outputs of the first and second summer circuit as aDFE equalized data sample.
 3. The clock recovery system of claim 1,wherein the DFFE circuit comprises: a DFFE summer circuit configured toadd a current DFE equalized data sample to each of a plurality ofcombinations of postcursor correction values and second symbol componentvalues to generate a plurality of summed output values; a DFFEmultiplexer circuit connected to outputs of the DFFE summer circuit; anda second component decode circuit configured to (i) determine a sign ofthe second symbol component of the current DFE equalized data sample,(ii) determine a sign of the second symbol component of a previous DFEequalized data sample, and (iii) output a control signal that indicatesthe determined signs of the second symbol components of the current andprevious DFE equalized samples; wherein the control signal output fromthe second component decode circuit controls the DFFE multiplexercircuit to select one of the summed output values as a DFFE equalizeddata sample.
 4. The clock recovery system of claim 3, wherein the DFFEsummer circuit comprises: a first summer circuit configured to output afirst summed value by adding values −LSB and −LSBH1 to the current DFEequalized data sample; a second summer circuit configured to output asecond summed value by adding values −LSB and +LSBH1 to the current DFEequalized data sample; a third summer circuit configured to output athird summed value by adding values +LSB and −LSBH1 to the current DFEequalized data sample; a fourth summer circuit configured to output afourth summed valued by adding values +LSB and +LSBH1 to the current DFEequalized data sample; wherein the LSBH1 value comprises a precomputedpostcursor correction value and the LSB values comprises a precomputedsecond symbol component value.
 5. The clock recovery system of claim 1,further comprising control logic to precompute postcursor correctionsvalues that are used by the DFE and DFFE circuits, and to precompute acomponent value for at least one of the symbol components that areremoved from the DFFE equalized data samples.
 6. The clock recoverysystem of claim 5, wherein the control logic is configured to process anoperating mode control signal to configure an operating mode of theclock recovery system for one of 2-level line modulation and 4-levelline modulation.
 7. The clock recovery system of claim 1, furthercomprising control circuitry configured to process the equalized 2-leveldigital signal to determine an amount of residual postcursor andprecursor ISI of the equalized 2-level digital signal, and to adjust aphase of the sample clock signal that is applied to the ADC circuit tominimize the residual postcursor and precursor ISI of the equalized2-level digital signal.
 8. The clock recovery system of claim 7, whereinthe control circuitry comprises: a sign error detection circuitconfigured to determine sign error values for the equalized 2-level datasamples and sign values of the equalized 2-level data samples; a phasedetector circuit configured to process the sign error values and thesign values of the equalized 2-level data samples to determineearly-late phase corrections signals; a loop filter circuit configuredto process the early-late phase correction signals and generate phaseadjustment control signals; and a phase adjustment circuit whichreceives the phase adjustment control signals from the loop filtercircuit, and which is configured to adjust the phase of the sample clocksignal that is applied to the ADC circuit to minimize the residualpostcursor and precursor ISI of the equalized 2-level digital signal. 9.A digital receiver system, comprising: an amplifier circuit configuredto amplify a received analog signal, wherein the analog signal comprisesa stream of data symbols, wherein each data symbol comprises a plurality(n) of symbol components, including at least a first symbol componentand a second symbol component; an ADC (analog-to-digital converter)circuit configured to sample the analog signal in response to a sampleclock signal and generate a digital signal comprising sequence ofdigital data samples, wherein each digital data sample corresponds toone data symbol in the analog signal; a digital equalization systemconfigured to filter the digital signal to remove intersymbolinterference and recover the stream of data symbols; a clock recoverysystem configured to process the digital signal and generate the sampleclock signal which is applied to the ADC circuit, wherein the clockrecovery system comprises: a FFE (feed forward equalizer) circuitconfigured to remove precursor ISI (intersymbol interference) from thedigital data samples and at least partially remove postcursor ISI fromthe digital data samples, and output FFE equalized data samples; a DFE(decision feedback equalizer) circuit configured to remove a firstcomponent of postcursor ISI from the FFE equalized data samples, andoutput DFE equalized data samples, wherein the first component ofpostcursor ISI of a given data sample arises from the first symbolcomponent of a previous data sample; and a DFFE (decision feedforwardequalizer) circuit configured to remove a second component of postcursorISI from the DFE equalized data samples, and generate DFFE equalizeddata samples, wherein the second component of postcursor ISI of a givendata sample arises from the second symbol component of a previous datasample; wherein the DFFE circuit is further configured to remove all butone of the n symbol components from each DFFE equalized data sample, andoutput an equalized 2-level digital signal comprising a sequence ofequalized 2-level data samples; and wherein the analog signal comprisesa 4-level PAM (pulse amplitude modulation) signal, wherein each datasymbol comprises a most-significant bit (MSB) as the first symbolcomponent, and a least significant bit (LSB) as the second symbolcomponent, and wherein the equalized 2-level digital signal output fromthe DFFE circuit comprises only the MSB components of the digital datasamples.
 10. The digital receiver system of claim 9, wherein the DFEcircuit of the clock recovery system comprises: a first summer circuitconfigured to add a postcursor correction value to a current FFEequalized data sample; a second summer circuit configured to subtractthe postcursor correction value from the current FFE equalized datasample; a multiplexer circuit connected to an output of each of thefirst and second summer circuits; and first component decode circuitconfigured to determine a sign of the first symbol component of aprevious FFE equalized data sample, and output a control signal thatindicates the determined sign of the first symbol component of aprevious FFE equalized data sample; wherein the control signal outputfrom the first component decode circuit controls the multiplexer circuitto select one of the outputs of the first and second summer circuit as aDFE equalized data sample.
 11. The digital receiver system of claim 9,wherein the DFFE circuit of the clock recovery system comprises: a DFFEsummer circuit configured to add a current DFE equalized data sample toeach of a plurality of combinations of postcursor correction values andsecond symbol component values to generate a plurality of summed outputvalues; a DFFE multiplexer circuit connected to outputs of the DFFEsummer circuit; and a second component decode circuit configured to (i)determine a sign of the second symbol component of the current DFEequalized data sample, (ii) determine a sign of the second symbolcomponent of a previous DFE equalized data sample, and (iii) output acontrol signal that indicates the determined signs of the second symbolcomponents of the current and previous DFE equalized samples; whereinthe control signal output from the second component decode circuitcontrols the DFFE multiplexer circuit to select one of the summed outputvalues as a DFFE equalized data sample.
 12. The digital receiver systemof claim 11, wherein the DFFE summer circuit comprises: a first summercircuit configured to output a first summed value by adding values −LSBand −LSBH1 to the current DFE equalized data sample; a second summercircuit configured to output a second summed value by adding values −LSBand +LSBH1 to the current DFE equalized data sample; a third summercircuit configured to output a third summed value by adding values +LSBand −LSBH1 to the current DFE equalized data sample; a fourth summercircuit configured to output a fourth summed valued by adding values+LSB and +LSBH1 to the current DFE equalized data sample; wherein theLSBH1 value comprises a precomputed postcursor correction value and theLSB values comprises a precomputed second symbol component value. 13.The digital receiver system of claim 9, wherein the clock recoverysystem further comprises control logic to precompute postcursorcorrections values that are used by the DFE and DFFE circuits, and toprecompute a component value for at least one of the symbol componentsthat are removed from the DFFE equalized data samples.
 14. The digitalreceiver system of claim 13, wherein the control logic is configured toprocess an operating mode control signal to configure an operating modeof the clock recovery system for one of 2-level line modulation and4-level line modulation.
 15. The digital receiver of claim 9, whereinthe clock recovery system further comprises control circuitry configuredto process the equalized 2-level digital signal to determine an amountof residual postcursor and precursor ISI of the equalized 2-leveldigital signal, and to adjust a phase of the sample clock signal that isapplied to the ADC circuit to minimize the residual postcursor andprecursor ISI of the equalized 2-level digital signal.
 16. The digitalreceiver system of claim 15, wherein the control circuitry of the clockrecovery system comprises: a sign error detection circuit configured todetermine sign error values for the equalized 2-level data samples andsign values of the equalized 2-level data samples; a phase detectorcircuit configured to process the sign error values and the sign valuesof the equalized 2-level data samples to determine early-late phasecorrections signals; a loop filter circuit configured to process theearly-late phase correction signals and generate phase adjustmentcontrol signals; and a phase adjustment circuit which receives the phaseadjustment control signals from the loop filter circuit, and which isconfigured to adjust the phase of the sample clock signal that isapplied to the ADC circuit to minimize the residual postcursor andprecursor ISI of the equalized 2-level digital signal.
 17. A method,comprising: receiving an analog signal, wherein the analog signalcomprises a stream of data symbols, wherein each data symbol comprises aplurality (n) of symbol components, including at least a first symbolcomponent and a second symbol component; converting the analog signalinto a digital signal by sampling the analog signal using a sample clocksignal to generate a sequence of digital data samples, wherein eachdigital data sample corresponds to one data symbol in the analog signal;equalizing the digital signal using a FFE (feed forward equalization)function to remove precursor ISI (intersymbol interference) from thedigital data samples and at least partially remove postcursor ISI fromthe digital data samples, and generate FFE equalized data samples;equalizing the FFE equalized data samples using a DFE (decision feedbackequalization) function to remove a first component of postcursor ISIfrom the FFE equalized data samples, and generate DFE equalized datasamples, wherein the first component of postcursor ISI of a given datasample arises from the first symbol component of a previous data sample;equalizing the DFE equalized data samples using a DFFE (decisionfeedforward equalization) function to remove a second component ofpostcursor ISI from the DFE equalized data samples, and generate DFFEequalized data samples, wherein the second component of postcursor ISIof a given data sample arises from the second symbol component of aprevious data sample; and removing all but one of the n symbolcomponents from each DFFE equalized data sample to generate an equalized2-level digital signal comprising a sequence of equalized 2-level datasamples; wherein the analog signal comprises a 4-level PAM (pulseamplitude modulation) signal, wherein each data symbol comprises amost-significant bit (MSB) as the first symbol component, and a leastsignificant bit (LSB) as the second symbol component, and wherein theequalized 2-level digital signal output from the DFFE circuit comprisesonly the MSB components of the digital data samples.
 18. The method ofclaim 17, wherein equalizing the FFE equalized data samples using theDFE function comprises: adding a postcursor correction value to acurrent FFE equalized data sample to generate a first sum value;subtracting the postcursor correction value from the current FFEequalized data sample to generate a second sum value; determining a signof the first symbol component of a previous FFE equalized data sample;generating a control signal that indicates the determined sign of thefirst symbol component of a previous FFE equalized data sample; andutilizing the control signal to select one of the first and second sumvalues as a DFE equalized data sample.
 19. The method of claim 17,wherein equalizing the DFE equalized data samples using the DFFEfunction comprises: adding a current DFE equalized data sample to eachof a plurality of combinations of postcursor correction values andsecond symbol component values to generate a plurality of summed outputvalues; determining a sign of the second symbol component of the currentDFE equalized data sample; determining a sign of the second symbolcomponent of a previous DFE equalized data sample; generating a controlsignal that indicates the determined signs of the second symbolcomponents of the current and previous DFE equalized samples; andutilizing the control signal to select one of the summed output valuesas a DFFE equalized data sample.
 20. The method of claim 17, furthercomprising processing the equalized 2-level digital signal to determinean amount of any residual postcursor and precursor ISI of the equalized2-level digital signal, and to adjust a phase of the sample clock signalto minimize the residual postcursor and precursor ISI of the equalized2-level digital signal.